Silane cation bifurcation time measured after photoionization

Fragmentation of silane molecules plays an important role in the plasma processes that create highly pure silicon layers in the production of photovoltaic cells or modern transistors. An international team, including Vít Svoboda from the Department of Physical Chemistry, UCT Prague, has now been able to describe the temporal evolution of silane cation after photoionization, thanks to a unique experiment and advanced theoretical molecular dynamics calculations. In addition to the aforementioned commercial applications, understanding ultrafast processes such as this one allows for a better understanding of the basic behaviour of molecules and the principles of chemical physics. The study was published in the respected Nature Communications journal.

Molecular symmetry is a fundamental concept in chemistry since it can be used to predict or explain many of a molecule’s chemical properties. Highly symmetrical molecules such as silane often break into fragments during Jahn-Teller disassociation (fission upon ionization). However, direct experimental evidence of the temporal evolution over which these changes occur has been lacking.

The international team managed to measure the temporal course of Jahn-Teller disassociation of the silane cation using short pulses (pulse duration <1 fs) during attosecond soft-X-ray-absorption spectroscopy. In the experiment, an infrared pulse first strong-field-ionized molecules in the gas phase. The researched monitored the course of the reaction by measuring the absorption spectra of the resulting reaction products with attosecond soft-X-ray probe pulses. The resulting 3D spectra provided information about absorption as a function of the reaction time and the energy of the given absorption band.

„To decipher all the details contained in the measured spectra, we simulated the entire photoionization reaction using advanced theoretical calculations,“ Dr. Svoboda said, adding that almost immediately after the ionization of the silane cation, two possible dissociation pathways opened up due to symmetry.

UCT Prague: Vit Svoboda

„The first path was the dissociation of the silane cation to SiH₃⁺, which occurred without an energy barrier and lasted 23 fs. The second path, which took place 11 fs later than the first path, was the stochastic decay of the silane cation to SiH₂⁺ and H₂, which lasted 140 fs. From the point of view of chemical dynamics, it is interesting to note how the trace of the silane molecule‘s vibration before ionization gets imprinted in the vibrational memory of the SiH₃⁺ fragments but not in the SiH₂⁺ fragments,“ Dr. Svoboda explained. He recently returned to UCT Prague, where he is establishing his own research group with the support of a Czech Science Foundation JUNIOR STAR grant and a grant from the Dagmar Procházková Fund. Previously, he spent time at ETH Zurich, where he received his doctorate, the Max Born Institute in Berlin, and JILA, a joint institute of the University of Colorado Boulder and the US National Institute of Standards &Technology (NIST).

The researchers hope that the results published in Nature Communications might contribute, in the near future, to the optimization of industrial processes, to higher quality semiconductor materials, and also lead to a deeper understanding of the ultrafast dynamics of molecules after photoionization.

The original article

Attosecond X-ray spectroscopy reveals the competing stochastic and ballistic dynamics of a bifurcating Jahn–Teller dissociation

Danylo Matselyukh, Vít Svoboda & Hans Jakob Wörner

Nat Commun 16, 6540 (2025)

https://doi.org/10.1038/s41467-025-61512-8

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Methods

Attosecond transient absorption spectroscopy

The ATAS experiment is driven with a 5.2 fs FWHM optical pulse spanning 500–1000 nm, generated by spectrally broadening the compressed output of a FEMTOPOWER V CEP titanium:sapphire laser system. This few-cycle pulse serves as both the pump pulse as well as the driving pulse for high harmonic generation (HHG). HHG is performed in helium to generate the attosecond soft-X-ray probe pulses exhibiting a continuous spectrum at the silicon L2,3-edge. The attosecond delay between the pump and probe pulses is controlled using a custom-built attosecond interferometer. The pump and probe pulses pass through the target gas (Silane 5.0 from Linde) in a 1 cm-long target cell. The transmitted X-ray spectra are measured using a Hitachi 001-0660 flat field grating and a Princeton Instruments PIXIS-XO 2KB X-ray camera. For more details see Supplementary Section S1.1.

Time-of-flight mass spectrometry

The second probe of pump-induced dynamics takes the form of a mass-spectrometry measurement. Here, it measures the charged fragments that result from the strong-field ionization of silane molecules by the pump pulse. This home-built in-situ TOF-MS is made up of a Photonis MegaSpiraltron channeltron, a grounded mesh, a reflection mesh and the target cell itself as
seen in Fig. S2.

Nat Commun 16, 6540 (2025): Figure S2: Mass Spectrometry of silane A. Schematic drawing of the interaction region showing the gas cell, focused laser beams (yellow), the electrode meshes used for the extraction of the ions and the channeltron detector. B. Exemplary mass spectrum obtained by strong-fieldionizing SiH4 under typical experimental conditions.

To perform mass spectrometry, the gas cell is positioned such that its bottom is a few millimeters above the pump-pulse focus (Fig. S2A). Next a very small amount of silane gas is passed through it into the vacuum chamber (yielding a pressure of 100, sufficient for resolving the 28SiH3 + and 28SiH2 + fragments, as well as their lower abundance 29Si and 30Si isotopologues (see Fig. S2B for details)